First half of the 20th century (World War I through World War II) In 1915, during World War I, the concept of crimes against humanity was introduced into international relations for the first time when the Allied Powers sent a letter to the government of the Ottoman Empire, a member of the Central Powers, protesting massacres that were taking place within the Empire.1915 declaration: ; ; : Status: Ordered to be Reported by the Yeas and Nays: 40 – 7.

Ottoman Empire

On 24 May 1915, the Allied Powers (Britain, France, and Russia) jointly issued a statement which for the first time ever explicitly charged a government, the Ottoman Empire, with committing a "crime against humanity" in reference to that regime's persecution of its Christian minorities, including Armenians, Assyrians and Greeks. Many researchers consider these events a single genocide rather than separate genocides, based on their belief that all of these genocides were part of the planned ethnoreligious purification of the Turkish state, a policy which was implemented and advanced by the Young Turks.

This joint statement stated, "[i]n view of these new crimes of Turkey against humanity and civilisation, the Allied Governments announce publicly to the Sublime Porte that they will hold personally responsible for these crimes all members of the Ottoman Government, as well as those of their agents who are implicated in such massacres."

Greeks The Greek genocide refers to the fate of the Greek population of the Ottoman Empire both during and after World War I (1914–18). Like the Armenians and the Assyrians, the Greeks were also subjected to massacres, expulsions, death marches and various other forms of persecution by the Young Turks.: "Already in the period 1912–14, the Young Turk leadership aimed to replace the multi-ethnic and multi-confessional.... The elimination of the Armenian, Assyrian, and Greek populations was an integral part of the Young Turk struggle for ..." The mass killing of Greeks continued to occur under the rule of the Turkish National Movement during the Greco-Turkish War phase of the Turkish War of Independence. George W. Rendel of the British Foreign Office, among other diplomats, documented the massacres and deportations of Greeks during the post-Armistice period. Estimates of the number of Anatolian Greeks who were killed range from 348,000 to 900,000.

Assyrians The Assyrian genocide (also known as the Sayfo or the Seyfo; Aramaic: ܩܛܠܐ ܕܥܡܐ ܐܬܘܪܝܐ or ܣܝܦܐ, ) was committed against the Assyrian population of the Ottoman Empire during the First World War by the Young Turks. The Assyrian population of northern Mesopotamia (Tur Abdin, Hakkâri, Van, Siirt region in modern-day southeastern Turkey and Urmia region in northwestern Iran) was forcibly relocated and massacred by Ottoman (Turkish and allied Kurdish) forces between 1914 and 1920. This genocide paralleled the Armenian genocide and Greek genocide. The Assyro-Chaldean National Council stated in a 4 December 1922, memorandum that the total death toll is unknown, but it estimated that about 750,000 Assyrians were murdered between 1914 and 1918.

Armenians The Armenian genocide (, translit.: ; ) refers to the deliberate and systematic destruction of the Armenian population of the Ottoman Empire which occurred both during and just after World War I. It was implemented through extensive massacres and deportations, with the deportations consisting of forced marches under conditions which were designed to lead to the death of the deportees. The total number of resulting deaths is generally held to have been between one and one and a half million.

The genocide began on 24 April 1915, when Ottoman authorities arrested some 250 Armenian intellectuals and community leaders in Constantinople. Thereafter, the Ottoman military uprooted Armenians from their homes and forced them to march hundreds of miles, without food or water, to the desert of what is now Syria. The Armenians were massacred regardless of their age or gender, with rape and other acts of sexual abuse being commonplace. The majority of Armenian diaspora communities were founded as a result of these events. Mass killings continued to be committed by the Republic of Turkey during the Turkish–Armenian War phase of the Turkish War of Independence.

Modern Turkey succeeded the Ottoman Empire in 1923 and since then, it has denied the fact that a genocide occurred. In recent years, it has resisted calls to acknowledge the crime by scholars, countries and international organisations.

Diyarbekir

In 1915, a genocide was committed in Diyarbekir vilayet, claiming the lives of most Armenians, Syriac Christians, Greek Orthodox, and Greek Catholics living there. The genocide was ordered by governor Mehmed Reshid, partly with the backing of the CUP Central Committee. According to historian David Gaunt, "These figures indicate that although the eradication of the Assyrian [Syriac] population was extreme, it was still not as total as for the Armenians." According to historian Uğur Ümit Üngör, "all Christian communities of Diyarbekir were equally hit by the genocide, although the Armenians were often particularly singled out for immediate destruction".

Yazidis

During the Armenian genocide, many Yazidis were killed by Hamidiye cavalry. According to Aziz Tamoyan, as many as 300,000 Yazidis were killed with the Armenians, while others fled to Transcaucasia.

Kurds

Concurrent to the Late Ottoman genocides most sources suggest that as many as 700,000 Kurds were deported during World War I, although there are no reliable statistics. Safrastian (1948) estimates that half of these deported Kurds died. Genocide scholar Üngör writes that "it would require a separate study to calculate meticulously how many were deported".

A few decades later deportations continued. The Dersim massacre for example refers to the depopulation of Dersim in Turkish Kurdistan, in 1937–38, in which approximately 13,000–40,000 Alevi Kurds were killed and thousands more of them were driven into exile. A key component of the Turkification process was a policy of massive population resettlement. The main document, the 1934 Law on Resettlement, was used to target the region of Dersim as one of its first test cases, with disastrous consequences for the local population.

Many Kurds and some ethnic Turks consider the events which took place in Dersim a genocide. A prominent proponent of this view is İsmail Beşikçi. Under international laws, the actions of the Turkish authorities were arguably not genocide, because they were not aimed at the extermination of a people, but at resettlement and suppression. A Turkish court ruled in 2011 that the events could not be considered genocide because they were not directed systematically against an ethnic group. Scholars such as Martin van Bruinessen, have instead talked of an ethnocide directed against the local language and identity.

Russia and the Soviet Union

Kyrgyz

In 1916, in territory which is currently part of Kazakhstan and Kyrgyzstan, an uprising against Tsarist Russia occurred. A public commission in Kyrgyzstan called the crackdown of 1916 in which 100,000 to 270,000 Kyrgyz were killed, a genocide, though Russia rejected this characterisation. Russian sources put the death toll at 3,000.

Pogroms against Jews

The Whitaker Report of the United Nations cited the massacre of 100,000 to 250,000 Jews in more than 2,000 pogroms which occurred during the White Terror in Russia as an act of genocide. During the Russian Civil War, between 1918 and 1921, a total of 1,236 pogroms were committed against Jews in 524 towns in Ukraine. Estimates of the number of Jews who were killed in these pogroms range from 30,000 to 60,000. Of the recorded 1,236 pogroms and excesses, 493 of them were carried out by Ukrainian People's Republic soldiers who were under the command of Symon Petliura, 307 of them were carried out by independent Ukrainian warlords, 213 of them were carried out by Denikin's army, 106 of them were carried out by the Red Army and 32 of them were carried out by the Polish Army.

Decossackisation

During the Russian Civil War the Bolsheviks engaged in a genocidal campaign against the Don Cossacks. University of York Russian specialist Shane O'Rourke states that "ten thousand Cossacks were systematically slaughtered in a few weeks in January 1919" and he also states that this mass-slaughter "was one of the main factors which led to the disappearance of the Cossacks as a nation." The late Alexander Nikolaevich Yakovlev, head of the Presidential Committee for the Rehabilitation of Victims of Political Repression, notes that "hundreds of thousands of Cossacks were killed". Historian Robert Gellately claims that "the most reliable estimates indicate that between 300,000 and 500,000 were killed or deported in 1919–20" out of a population of around three million.

Peter Holquist states that the overall number of executions which were carried out is difficult to establish. In some regions hundreds were executed. In Khoper, the tribunal was very active, with a one-month total of 226 executions. The Tsymlianskaia tribunal oversaw the execution of over 700 people. The Kotel'nikovo tribunal executed 117 in early May and nearly 1,000 were executed overall. Others were not quite as active. The Berezovskaia tribunal made a total of twenty arrests in a community of 13,500 people. Holquist also notes that some of the White reports of Red atrocities in the Don were consciously scripted for agitation purposes. In one example, an insurgent leader reported that 140 were executed in Bokovskaia, but later provided a different account, according to which only eight people in Bokovskaia were sentenced to death, and the authorities did not manage to carry these sentences out. This same historian emphasises he is "not seeking to downplay or dismiss very real executions by the Soviets".

Research by Pavel Polian from the Russian Academy of Sciences on the subject of forced migrations in Russia shows that more than 45,000 Cossacks were deported from the Terek province to Ukraine. Their land was distributed among pro-soviet Cossacks and Chechens.

Ingrian Finns

The genocide of the Ingrian Finns () was a series of events triggered by the Russian Revolution in the 20th century, in which the Soviet Union deported, imprisoned and killed Ingrians and destroyed their culture. In the process, Ingria, in the historical sense of the word, ceased to exist. Before the persecution there were 140,000 to 160,000 Ingrians in Russia and today approximately 19,000 (including several thousand repatriated since 1990).

Joseph Stalin